The present invention relates to an apparatus and a method for producing an original plate for optical discs, and particularly to an apparatus and a method for producing an original plate for optical discs by applying laser light on a photosensitive layer or a surface layer formed on an original plate.
An optical disc such as a CD (trade name, Compact Disc) or DVD (trade name, Digital Versatile Disc) is used to record a digital signal converted from an audio signal or a video signal by forming pits composed of fine irregularities in a surface layer of the optical disc along the circumferential direction; or it is used as an external storage medium for a computer. Such an optical disc is formed from a resin, for example, polycarbonate resin by injection-molding.
An original plate called a stamper is used for injection-molding such an optical disc. The original plate is formed of a circular metal plate having irregularities reversely corresponding to fine irregularities called pits to be formed in the disc. The original plate is produced by a producing apparatus which will be described in the following.
FIG. 13 shows a principle of such an apparatus for producing an original plate for optical discs. The apparatus includes a laser light generator 1; a fixed diaphragm 3 for limiting parallel rays 2 of laser light generated by the laser light generator 1; a mirror 4 for reflecting downward the parallel rays 2 limited by the fixed diaphragm 3; and an objective lens 5 for refracting the parallel rays 2 reflected from the mirror 4 in such a manner that the parallel rays 2 are focussed on an original plate 6. The original plate 6, disposed under the objective lens 5, has a photoresist layer 7 formed on the original plate 6.
The surface of the original plate 6 is washed by a washer, and is coated with the photoresist layer 7 having a thickness of about 100 nm by a resist coater. A signal is recorded in the photoresist layer 7 formed on the original plate 6 by applying a laser beam on the photoresist layer 7 through the objective lens 5. Specifically, the signal is recorded in the photoresist layer 7 by chemically changing, using laser light, a portion of the photoresist layer 7 on which the laser light is applied, or directly processing the original plate 6 using an energy of laser light.
In the case of chemically changing the photoresist layer 7, the original plate 6 is completed by developing the photoresist layer 7 by a developer. Then, a nickel thin film is formed on the original plate by sputtering or electroless plating using a thin film forming system, followed by plating of nickel using the thin film as an electrode to a thickness of about 300 .mu.m by an electroforming system. A nickel sheet is stripped from the original plate 6, followed by removal of the remaining photoresist. Then, inside and outside diameters of the nickel sheet are adjusted depending on the kind of an optical disc to be produced, to complete a stamper. The dimensions of the stamper are shown in Table 1.
TABLE 1 ______________________________________ General Diameter of Stamper kind of medium inside diameter outside diameter ______________________________________ CD about 37.6 mm about 138 mm MD about 22.0 mm about 70 mm LD about 67.0 mm about 340 mm ______________________________________
In the above steps of producing an original plate for optical discs, the producing condition is carefully determined such that optical discs produced using the original plate in a plurality of subsequent steps satisfy a standard. Besides, in the case of producing different kinds of media (optical discs), since these media are different from each other in terms of pit shape, size of original plate, and the like, the producing condition is required to be set for each medium. For example, a CD is different from a DVD in terms of shape and size of pit, as in Table 2.
TABLE 2 ______________________________________ Comparison in Pit Size between CD and DVD CD DVD ______________________________________ pit length 0.9-3.3 .mu.m 0.4-1.87 .mu.m pit width 0.5-0.8 .mu.m 0.4 .mu.m or less track pitch 1.6 .mu.m 0.74 .mu.m capacity 780 Mbyte 4.7 Gbyte (one surface/ single layer) ______________________________________
In the apparatus for producing an original plate for optical discs, various kinds of optical components must be adjusted at high accuracies. For this reason, only one producing condition has been set for one apparatus specialized for one kind of original plate for optical discs. In other words, to produce different kinds of optical discs, it has been required to prepare a plurality of apparatuses specialized for the different kinds of original plates for optical discs. Alternatively, to produce a plurality of kinds of original plates for optical discs using only one apparatus, optical devices and optical components (a laser system, a series of lenses, and a diaphragm) in the producing apparatus have been required to be exchanged or adjusted depending on the kinds of the original plates for optical discs to be produced.
In the case in which different original plates for optical discs are produced using a plurality of apparatuses specialized for the different original plates for optical discs, there occur disadvantages that the cost of equipment becomes higher and also the installation area becomes larger. On the other hand, in the case in which different original plates for optical discs are produced by exchanging or adjusting optical devices in one producing apparatus, there occur disadvantages that it takes a lot of time for exchange or adjustment of the optical devices and also it is difficult to guarantee a stable performance after exchange of the optical devices.